Beneficial agents of various kinds can be administered to a living being in a variety of ways. Frequently, they are administered in some type of tablet or capsule. These are popular means of administration, for they are both convenient and easy to use. In their most simple form, tablets and capsules comprise a homogenous mixture of the beneficial agent and a carrier of some sort. Typically, the beneficial agent is released fairly rapidly from these forms, leading to an initial high concentration of the beneficial agent in the body which rapidly decreases.
Frequently, it would be advantageous to deliver a beneficial agent to a living being over a prolonged period of time and, preferably, at a substantially steady rate. A variety of devices are known for the sustained release of beneficial agents. In some such devices, the agent is dispersed in a polymer matrix which may be permeable to the agent by diffusion or which may be microporous, the pores containing an agent-permeable medium such that the agent will preferentially dissolve in and permeate through that medium.
Other devices are characterized by a reservoir which holds the agent and, perhaps, a carrier, and which is surrounded by a wall or barrier of some sort. The wall may be a semipermeable material that allows body fluid into the reservoir but does not allow the beneficial agent to exit. In such a device there may be a passageway in the wall which allows the solution of agent-containing body fluid to pass from the device into the tissues of the body. Alternatively, the wall may be impermeable in part and permeable in part, such that the beneficial agent can diffuse through the permeable portion over time.
As indicated above, a wide variety of devices have been suggested or designed in recent years. A frequent shortcoming of these devices, however, has been either that they could not be used to obtain a substantially steady rate of release of the beneficial agent over time (otherwise described as a rate of release approaching zero order) or, if they did provide a substantially steady release rate, they were difficult and time-consuming to construct. For example, U.S. Pat. No. 4,289,795, issued to Bogentoft et al., discloses a method for preparing a delivery system having controlled release of an active component wherein a particle is coated in a continuous coating operation with a composition which comprises the active component and an inactive release-controlling substance. The concentration of the active component is decreased as the coating operation progresses. Such a method, which calls for repeated coating steps and changes in the composition of the coating material during the coating process, is impractical for large-scale, commercial use.
Accordingly, it is an object of this invention to provide a device for the sustained release of a beneficial agent that is relatively easy to make and provides for a substantially steady rate of release of the agent.
It is a further object of this invention to provide such a device that can be used with a wide variety of beneficial agents.
It also is an object of the present invention to provide such a delivery device in a variety of sizes and shapes and in a form suitable for oral ingestion, implantation into the tissues of a body or insertion into a body cavity.
Additional objects of the present invention shall become apparent to those skilled in the art from the description of the invention below taken in conjunction with the accompanying claims and figures.